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Description
The Picacho area of SE California is underlain by an extensional duplex consisting of four major plates. From oldest to youngest the duplex consists of (1) the Late Cretaceous to Early Tertiary Orocopia Schist, (2) probable Jurassic granodioritic gneiss, (3) probably Jurassic Winterhaven Formation, and (4) Cenozoic volcanic and sedimentary rocks. The oldest unit in structural unit (4) is the Oligocene Quechan Volcanics. The Quechan Volcanics were named by Crowe ( 1978) who described them as being ~300 meters thick. According to Crowe (1978) the lower part consists of ~30 to ~50 meters of trachybasalt, overlain by a thick section of pyroxene rhyodacite which was in turn overlain by glassy pyroxene dacite. However, Andrea Rowland-Smith has documented that the Quechan Volcanics in the Picacho area consists instead of a lower ~50 m of alkali basalt and an upper ~250 m oftrachybasalt. In order to explore further the composition of the Quechan Volcanics an additional 9 samples were collected from a location approximately 6.5 km along strike from the section sampled and studied by Rowland-Smith. Field observations indicate that the Quechan Volcanics within the studied section are composed of flows varying from ~20 to 30 meters in thickness. Vesiculated and brecciated bases and tops of some flows were noted. Compositionally, the lower ~50 meters is composed of pyroxene porphyry, while the upper ~250 meters is composed of pyroxene-plagioclase porphyry. Thin section study also shows that the cores of plagioclase phenocrysts are completely replaced by calcite, and that both quartz and calcite are common infillings of vesicles. The introduction of calcite and quartz and the extensive fracturing of the sampled section are likely due to its proximity to a young high silica rhyolite plug that introduced hydrothermal circulation cells. New major and trace element chemical data confirm the work of Rowland-Smith even after alteration.
